1. Field of the Invention
The present invention relates to a solid state image sensing device, and more particularly, to a charge coupled device (CCD) type solid state image sensing device.
2. Description of the Related Art
A charge-coupled device (CCD) is a semiconductor device that converts light into electronic bits of information. In essence, it is digital film. Charge-Coupled Device (CCD) based solid state imaging devices are commonly used in digital still cameras, mobile phone cameras and optical scanners, as well as in photocopiers, fax machines, and more recently, in digital video cameras, to capture images. CCDs are also widely used as sensors for astronomical telescopes, and night vision devices. A charge-coupled device (CCD), is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbors.
A CCD may operate as an optical transducer, a sensor with an electrical output that ideally is proportional to the intensity of illumination on the sensor itself. A CCD senses the intensity of light from a real image focused onto its pixel elements, converts the light into electric charges (signals) proportional to the intensity of the light, and transmits the signals to other devices, such as a memory, a printer, or to a display. A display driving apparatus receiving the signals from a CCD processes signals of color image data (R, G, and B) output from the solid state CCD device and drives a display apparatus such as a liquid crystal display (LCD).
FIG. 1 is a block diagram of a color image sensing system employing an conventional CCD based solid state imaging device. Referring to FIG. 1, a conventional color image sensing system comprises a CCD based solid state image sensing device 110, a frame memory buffer 120, an interpolator 130, and a color signal processing unit 140. The CCD based solid state image sensing device 110 outputs R, G, R, G, . . . , as first phase signals, and outputs G, B, G, B, . . . , as second phase signals. A full frame is composed of the interlaced combination of the first and second phase signals. The charge from each phase of the imaging CCD is transferred to another CCD array called the frame storage array (frame memory buffer). When full frame signals are output in this manner, both of the two phases of two-color signals are stored in the frame memory buffer 120 and then, (R, G) row signals and (G, B) row signals are output alternately by the frame memory. If the three-color signals (R, G, and B) are thus output in each predetermined (frame) period, the interpolator 130 performs interpolation of the three-color signals (R, G, and B), and the interpolated three-color signals (R, G, and B) of each frame are output to a display apparatus, through the signal processing unit 140.
FIG. 2 is a diagram showing the pixel structure of the CCD-based solid state image sensing device 110 of FIG. 1. The CCD-based solid state image sensing device 110 is driven by “Interline Transfer” method, where CCD itself has a electronic shutter. That means the CCD shifts each pixel (photodiode charge) into shift registers (vertical CCDs). These CCDs don't require a mechanical shutter, because can be controlled with electronics around each photodiode. The light is amplified with micro lenses.
The Interline Transfer CCD has the pixel structure shown in FIG. 2 including a two-dimensional photodiode matrix 260 and a vertical CCD connected to each of photodiodes 260. The pixels of the matrix work initially as light detectors then as shift register during charge transfer. The horizontal CCD works only as a shift register. Typically, in the case of a color CCD-based solid state image sensing device 110, a color filter is installed on the top of each pixel element so that light of only a predetermined color (e.g., one of G, B, or R) can be sensed (received). In order to form three-color signals, at least 3 types of color filters are disposed over the matrix of photodiodes 260. A most widely used color filter array is Bayer pattern, which is shown in FIG. 2. Two-color patterns of red (R) and green (G) are disposed in one row; and two-color patterns of green (G) and blue (B) are disposed in the other (alternate) row and these alternating two-color pattern rows are repeated. Each square of four pixels has one filtered red, one blue, and two green pixels. The color green (G) is closely related to a luminance signal and is therefore present in all rows; and the colors red (R) and blue (B) are disposed diagonally to maximize luminance resolution. Current digital still cameras, including CCD based solid state image sensing devices with millions of pixels (Megapixels) have achieved resolutions even higher than the effective resolution of consumer-grade 35 millimeter photographic film. Digital microscope cameras based on CCDs can capture extremely high resolution images, e.g., equivalent to 12.5 million pixels.
The conventional CCD-based solid state image sensing device 110 is designed to employ an interlaced pixel method, and each vertical CCD 250 employs a four-state driving signal (V1˜V4) method. During one phase, one of two pixels adjacent to each other in the vertical direction can transfer a signal charge to the CCD 250. The output signals of photodiodes 260 in all rows are not output at the same time and a method by which signals of every-other row of the photodiodes 260 is transmitted to the vertical CCD 250 during each phase, is used.
FIG. 3 is a flowchart explaining the steps in the operation of the CCD-based solid state image sensing device 110 of FIG. 2, and FIGS. 4a and 4b are diagrams to help explain of the operation of the CCD-based sold state image sensing device of FIG. 2. Referring to FIG. 3, in first step S310, the mechanical shutter is opened so that signal charges are accumulated in the photodiodes 260 for a predetermined time interval. Next, in step S320, if an active signal for readout is applied to a driving electrode 210 transferring V1 among the vertical driving signals, the video signals of predetermined rows (e.g., R, G rows) are transferred from the photodiodes 260 to the vertical CCD 250, and V1 charge packet thus transferred to the vertical CCD 250 is vertically transmitted by vertical driving signals (V1˜V4) in the vertical CCD (VCCD) 250 towards the horizontal CCD 270 (FIG. 4A). In step S330, the horizontal CCD (HCCD) 270 receives V1 charge packet, and horizontally transmits it by horizontal driving signals (VH1 and VH2) such that video signals of (R and G) rows are output. Likewise, in step S340, if in the next field phase (period), an active signal for readout is applied to the driving electrode 230 transferring V3 among vertical driving signals, and signals of predetermined rows (G, B rows) are transferred from the photodiodes 260 to the vertical CCD 250, and V3 charge packet thus transferred to the vertical CCD 250 is vertically transmitted in the vertical CCD 250 by vertical driving signals (V1˜V4) towards the horizontal CCD 270. In step S350, the horizontal CCD 270 receives V3 charge packet ((G, B) row signal packet in FIG. 4a ), and horizontally transmits by horizontal driving signals (VH1 and VH2) such that the video signals of (G, B) rows are output.
When the conventional CCD-based solid state image sensing device 110 outputs image signals of one frame in this method, an (R, G) row signal is output in the first phase (period), and a (G, B) row signal is output in the second phase (period). Accordingly, in order for the interpolator 120 to perform interpolation (to reproduce clear color signals), the frame memory buffer 120 is required. Thus, in order to output the three-color signals (R, G, and B) required by the interpolator 120, the frame memory buffer 120 stores one frame composed of signals formed by combining the first and second phase image signals output from the solid state image sensing device 110, and then outputs three-color signals (R, G, and B). Accordingly, in a color image sensing system employing this conventional CCD-based solid state image sensing device 110 and the method driving the device 110, a large capacity of memory is required, which consumes electrical power and takes up space, and it is disadvantageous or uneconomical for small-sized systems for mobile use, such as digital still cameras.